Method for reduction and separation of selenium in acid sludge wet leaching solution

Abstract

The application relates to the technical field of non-ferrous metal smelting and recycling, and discloses a method for reducing and separating selenium in acid sludge wet leaching liquid, which comprises the following steps: acid sludge chlorination leaching and rough selenium residue and post-depletion liquid generated by a step of reducing rough selenium from the leaching liquid; treating the post-depletion liquid through a neutralization and mercury precipitation step; mixing the rough selenium residue and the post-depletion liquid to perform alkaline leaching, adding NaOH to adjust the mass concentration to 10-20%, adding 1.05-1.20 times of the required amount of hydrazine hydrate under normal temperature and pressure, and separating an Se 2‑ alkaline leaching liquid; adding 1.0-1.15 times of the required amount of hydrogen peroxide to the Se 2‑ alkaline leaching liquid to perform oxidation and precipitate selenium, and separating a powder selenium product. Through the application, the side reaction dissolution of lead impurities is effectively inhibited, efficient separation of selenium and lead and mercury is realized, reagent consumption is reduced by combining with mother liquor circulation, and finally a high-purity powder selenium product is prepared.

Description

Technical Field

[0001] This invention relates to the field of non-ferrous metal smelting and recycling technology, specifically to a method for the reduction and separation of selenium in acid mud wet leaching solution. Background Technology

[0002] Acid sludge is a selenium-containing byproduct generated during non-ferrous metal smelting and sulfuric acid production. With increasing environmental protection requirements, wet processing techniques for treating acid sludge and extracting selenium have gained popularity due to their environmental friendliness. However, existing wet processes struggle to completely separate selenium from associated heavy metal impurities such as lead and mercury when treating acid sludge leachate.

[0003] In the purification and extraction of crude selenium, existing technologies typically employ alkaline leaching. To improve the solubility of selenium, conventional processes often involve leaching at high temperatures. This high-temperature, strongly alkaline system can cause side reactions in lead-containing phases such as lead sulfate in the crude selenium residue, leading to lead dissolution into the liquid phase and ultimately resulting in excessive lead content in the reduced selenium product. Simultaneously, free mercury ions present in the acid sludge leachate readily undergo reduction reactions with selenium and co-precipitate during conventional reduction removal processes, causing heavy metal contamination in the selenium product. Furthermore, conventional processes lack precise quantitative control of the amount of mercury removal agents added based on the reaction mechanism during impurity removal, easily leading to incomplete mercury removal or the introduction of new impurities due to excessive agent dosage.

[0004] Furthermore, the amounts of oxidant and reductant added in each step of the existing separation process are largely adjusted empirically, without precise matching to the theoretical consumption of elemental selenium or impurity ions. This leads to an unstable reaction system, high reagent consumption, and a low direct selenium recovery rate. The alkaline and selenium-containing intermediate filtrate generated in the process is typically discharged directly as wastewater, resulting in waste of chemical reagents such as sodium hydroxide and loss of some dissolved selenium, as well as increased wastewater treatment costs. These problems limit the efficient and high-purity recovery of selenium resources from acid sludge. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for the reduction and separation of selenium in acid mud wet leaching solution. This method solves the problems of incomplete separation of selenium from impurities such as lead and mercury during acid mud wet leaching, the tendency of traditional high-temperature leaching systems to cause lead phase to undergo side reactions and dissolve, resulting in low purity of crude selenium, and the inaccurate control of reagent dosage during the reduction and separation process, leading to unsatisfactory leaching rate and product purity.

[0006] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for the reduction and separation of selenium in wet leaching solution of acid mud, comprising the following steps: Chlorination leaching steps for acid mud: Mix raw acid mud with hydrochloric acid-containing bottom liquid, add sodium chlorate solid to react, and separate the solid and liquid to obtain crude selenium slag and qualified leachate; Leachate reduction of crude selenium step: Sodium sulfite crystals are added to qualified leachate for reaction, and the crude selenium residue and deselenized liquid are obtained by filtration; Neutralization and mercury precipitation steps: Add NaOH to the selenium-depleted solution to adjust the pH and produce neutralization residue. After filtration, add hydrazine hydrate liquid dropwise to the neutralized solution to react. Filter to obtain the mercury-depleted solution. Alkaline leaching steps for crude selenium slag: Combine the crude selenium slag obtained from the previous two steps and mix it with the deselenate-removed liquid. Add NaOH to adjust the mass concentration of NaOH in the final solution to 10% to 20%. Add hydrazine hydrate liquid dropwise at room temperature and pressure. The amount of hydrazine hydrate liquid added is controlled to reduce elemental selenium to Se. 2- The amount of Se was increased to 1.05 to 1.20 times the theoretical required amount, and the reaction was stirred for 1.5 to 2.5 hours, followed by pressure filtration to obtain Se-containing products. 2- The alkaline leachate; Oxidation precipitation of selenium: To the Se-containing... 2- Hydrogen peroxide was slowly added dropwise to the alkaline leachate for precipitation treatment. The amount of hydrogen peroxide added was controlled to reduce the amount of Se... 2- The amount of elemental selenium is oxidized to 1.0 to 1.15 times the theoretical amount required for elemental selenium, and then centrifuged to obtain powdered selenium products.

[0007] By adopting the above technical solution, and through a multi-stage synergistic process involving acid mud chlorination leaching, leaching solution reduction of crude selenium, neutralization and mercury precipitation, alkaline leaching of crude selenium slag, and oxidative selenium precipitation, and by strictly controlling the NaOH mass concentration and the excess coefficient of hydrazine hydrate during the alkaline leaching stage, an effective method is achieved to avoid the dissolution of lead impurities in the high-temperature leaching system and to realize the transformation of solid elemental selenium into soluble liquid Se. 2- The effect of unidirectional conversion and final preparation of high-purity powdered selenium products.

[0008] The specific reaction process and principle are as follows: In the first step, during the acid sludge chlorination leaching process, the strong oxidizing property of sodium chlorate in the hydrochloric acid base solution is used to oxidize the associated selenium in the acid sludge into selenite, which then enters the liquid phase, thus initially achieving solid-liquid separation of selenium from some insoluble impurities.

[0009] In the second step, during the reduction of crude selenium in the leachate, sodium sulfite crystals are added as a reducing agent to reduce selenite acid in the liquid phase back to elemental selenium and form crude selenium slag.

[0010] In the third step, during the neutralization and mercury precipitation process, the pH of the solution is first adjusted to 6.5 to 8.0 by adding NaOH, causing impurity ions such as iron and bismuth in the solution to form neutralization residue precipitates. Then, the reducing properties of hydrazine hydrate are used to selectively reduce divalent mercury ions in the solution to insoluble mercurous chloride precipitates, thereby achieving the removal of mercury from the system.

[0011] In the fourth step, during the alkaline leaching of the crude selenium slag, the deselenized liquid is recycled as mother liquor, and NaOH is added to maintain the final solution concentration in a strongly alkaline range of 10% to 20%. Under these strongly alkaline conditions, hydrazine hydrate is added dropwise at 1.05 to 1.20 times the theoretically required amount. The hydrazine hydrate provides reduction electrons, reducing all zero-valent elemental selenium in the crude selenium slag to soluble Na₂Se. Simultaneously, this step is carried out at room temperature and pressure. Combined with the set NaOH concentration and the excess ratio boundary of hydrazine hydrate, the side reaction dissolution of lead phases such as lead sulfate in the crude selenium slag can be suppressed, allowing lead to remain in the solid phase as residue, thus achieving the separation of selenium and lead.

[0012] In the fifth step, during the selenium oxidation precipitation, hydrogen peroxide in a controlled ratio is added dropwise to the alkaline leachate containing Na₂Se. The oxidizing property of hydrogen peroxide re-oxidizes the divalent selenium ions to zero-valent elemental selenium, which then precipitates out. Since lead, mercury, and other impurities have already been removed in the previous steps, the precipitated elemental selenium at this stage has a high purity. After centrifugation, the final powdered selenium product is obtained.

[0013] Preferably, in the alkaline leaching step of the crude selenium slag, NaOH is added to adjust the mass concentration of NaOH in the final solution to 15%, and the amount of hydrazine hydrate added is controlled to reduce elemental selenium to Se. 2- 1.12 times the theoretically required amount.

[0014] By adopting the above technical solution, and by limiting the specific strong alkaline environment and the amount of reducing agent, the reduction reaction rate and reagent consumption can be balanced, avoiding the introduction of excess impurities or the violent foaming of the system by excessive hydrazine hydrate, thus ensuring the dissolution and conversion rate of elemental selenium.

[0015] Preferably, the acid sludge chlorination leaching step includes loading the raw acid sludge into a leaching reactor, pumping in a circulating bottom liquid containing hydrochloric acid at a liquid-to-solid ratio of 2.5:1 to 4.0:1, and starting stirring at room temperature to 80°C; in the acid sludge chlorination leaching step, sodium chlorate solid is slowly added in batches for leaching, the amount of sodium chlorate added is controlled to be 1.05 to 1.15 times the theoretically required amount, and the leaching reaction time is controlled to be 1.5 h to 3.0 h.

[0016] By adopting the above technical solution, setting the liquid-to-solid ratio and temperature range, and adding a slightly excessive amount of sodium chlorate in batches, the oxidation potential of the system can be controlled, avoiding the large-scale escape of gas due to excessively high instantaneous concentration of oxidant, while ensuring that the selenium-containing phase wrapped in the acid sludge is fully released and oxidized and dissolved.

[0017] Preferably, the step of reducing crude selenium with leachate includes pumping qualified leachate into a reduction reactor, and slowly adding sodium sulfite crystals at 40°C to 65°C with stirring to carry out the reduction reaction; the amount of sodium sulfite crystals added in the step of reducing crude selenium with leachate is controlled to be 1.15 to 1.25 times the amount theoretically required to completely reduce H2SeO3 to elemental Se, and solid-liquid separation is carried out after reacting for 1.0 h to 2.0 h.

[0018] By adopting the above technical solution, under medium-temperature heating and stirring conditions, an excess of sodium sulfite can be added to promote the complete reduction of selenite acid to precipitate as crude selenium slag, preventing unreacted selenium ions in the liquid phase from entering the subsequent removal process and causing a decrease in recovery rate.

[0019] Preferably, the neutralization and mercury precipitation steps include: draining the selenium-depleted solution into a reaction vessel; slowly adding NaOH tablets to adjust the pH of the solution to 6.5 to 8.0; centrifuging and filtering the resulting neutralization residue to obtain the neutralized solution; and slowly adding Hg from a hydrazine hydrate liquid reduction solution to the neutralized solution. 2+ Hg₂Cl₂ precipitate is formed, and the amount of hydrazine hydrate liquid added is controlled to reduce Hg₂Cl₂ precipitate. 2+ Reduce to 1.0 to 1.10 times the theoretically required amount of Hg2Cl2.

[0020] By adopting the above technical solution and using a stepwise precipitation method, heavy metals and amphoteric metal impurities are first removed by pH adjustment, and then free divalent mercury is reduced to mercurous chloride precipitate with extremely low solubility by controlling the reaction equivalent with hydrazine hydrate, thus preventing mercury from entering subsequent processes and improving the environmental safety of the system.

[0021] Preferably, in the selenium oxidation precipitation step, the selenium-containing... 2- Selenium precipitation was carried out by adding 27.5% hydrogen peroxide by mass to the alkaline leachate and controlling the reaction time to be 1.5h to 2.5h.

[0022] By adopting the above technical solution, using hydrogen peroxide of conventional concentration as the oxidant, and controlling the reaction time within an appropriate range, the oxidation process of divalent selenium ions can be ensured to be sufficient and the system stable. The precipitated elemental selenium crystals are intact, which facilitates solid-liquid separation.

[0023] Preferably, in the selenium oxidation precipitation step, the filtrate produced by centrifugation is partially or entirely recycled back to the alkaline leaching step of the crude selenium residue as mother liquor.

[0024] By adopting the above technical solution, the filtrate containing residual alkalinity after the precipitation of elemental selenium is recycled, unreacted sodium hydroxide and residual selenium ions are recovered and utilized, reagent consumption costs are reduced, waste liquid discharge is reduced, and the overall clean production level of the process is improved.

[0025] This invention provides a method for the reduction and separation of selenium in wet leachate of acid mud. It has the following beneficial effects: 1. This invention achieves efficient separation of selenium and lead by combining an alkaline leaching step in crude selenium slag with ambient temperature and pressure reaction conditions. Under a NaOH concentration of 10% to 20% and ambient temperature, and with 1.05 to 1.20 times the theoretical amount of hydrazine hydrate, solid elemental selenium can be completely converted into soluble sodium selenide and enter the liquid phase. At the same time, it inhibits the side reaction dissolution of lead-containing impurities such as lead sulfate in the crude selenium slag under a strong alkaline environment, so that lead is retained in the form of solid residue. Compared with the traditional high-temperature alkaline leaching process, this method avoids the simultaneous leaching of lead with selenium, reduces the subsequent purification load from the source, and improves the purity of the final selenium product.

[0026] 2. This invention employs a stepwise impurity removal strategy in the neutralization and mercury precipitation steps, effectively controlling mercury contamination and improving process safety. By adjusting the pH to 6.5 to 8.0, impurities such as iron and bismuth are removed in advance. Subsequently, the reducing properties of hydrazine hydrate are utilized to directionally reduce divalent mercury ions in the solution to insoluble mercurous chloride precipitate under precisely controlled addition, thereby removing mercury from the liquid phase system. This step is set before the alkaline leaching of crude selenium slag, preventing mercury from entering the subsequent selenium dissolution and reprecipitation processes, ensuring that the mercury content in the final powdered selenium product meets relevant standards.

[0027] 3. This invention reduces production costs and waste discharge by using a mother liquor circulation system and precise ratio of redox dosage within the process. The filtrate produced in the oxidation and selenium precipitation step contains residual alkalinity and unreacted selenium components, which is directly recycled back to the alkaline leaching step of the crude selenium slag as a preparation mother liquor, thus realizing the recovery and utilization of sodium hydroxide and selenium resources. At the same time, the use of low-multiple excess control of reagents such as sodium chlorate, sodium sulfite, hydrazine hydrate, and hydrogen peroxide based on theoretical calculations in each step ensures complete reaction while avoiding reagent waste and impurity introduction, thereby improving the selenium recovery rate of the entire process. Attached Figure Description

[0028] Figure 1 The following are statistical charts showing the physicochemical test results of the mass fraction of core elements in the products of each key process node in Examples 1 to 3: (a) is a gray-scale bar chart comparing the content of trace mercury impurities in crude selenium slag with the content of trace selenium residue in lead slag; (b) is a gray-scale bar chart comparing the enriched content of selenium in crude selenium slag with the enriched content of lead in lead slag; and (c) is a line graph showing the mass fraction of the final precipitated powder selenium product. Figure 2 The following are statistical charts showing the physicochemical test results of the key product recovery and conversion indicators and by-product purity in Examples 1 to 3: (a) is a gray-scale bar chart comparing the crude selenium alkaline leaching rate and the selenium precipitation single-step crystallization rate; (b) is a gray-scale broken line chart showing the whole process selenium direct recovery rate and the purity of mercurous chloride by-product. Figure 3 The following are statistical charts showing the physicochemical test results of the final products of Example 2 and Comparative Examples 1 and 2: (a) is a gray-scale bar chart comparing the total selenium mass fraction in the final powder selenium product, and (b) is a gray-scale cluster bar chart comparing the mass fraction of iron and lead impurities in the final powder selenium product. Figure 4 The following are statistical charts showing the physicochemical test results of selenium leaching and recovery indicators in Example 2, Comparative Example 4, and Comparative Example 5: (a) is a gray-scale clustered columnar comparison chart of the mass fraction of total selenium in crude selenium slag and residual selenium in lead slag; (b) is a gray-scale clustered columnar comparison chart of the alkaline leaching rate of crude selenium and the direct selenium recovery rate of the whole process. Figure 5 The following are statistical charts showing the oxidation precipitation index and liquid phase residue test results of Examples 2, 3 and Comparative Example 3: (a) is a gray-scale clustered columnar comparison chart of the single-step crystallization rate of selenium precipitation and the purity of the powder product; (b) is a logarithmic coordinate gray-scale broken line graph of the residual selenium concentration in the precipitate mother liquor. Detailed Implementation

[0029] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] Examples 1-3: Example 1

[0031] This embodiment provides a method for the reduction and separation of selenium in the wet leaching solution of acid mud. The raw material acid mud used in this batch has a selenium mass fraction of 5.2%, a mercury mass fraction of 2.1%, and a lead mass fraction of 45.3% on a dry basis. The method includes the following steps: (1) Chlorination leaching of acid sludge: The raw acid sludge (granular solid, no pretreatment required) is directly loaded from the raw material warehouse into the leaching reactor in the chlorination workshop. A strongly acidic bottom solution prepared with hydrochloric acid and condensate is pumped in at a liquid-to-solid ratio of 2.5:1. Stirring is started at room temperature, and sodium chlorate (NaClO3) solid is added slowly in batches for leaching. The amount of sodium chlorate added is 1.05 times the theoretically calculated amount. The leaching reaction time is controlled at 1.5h. After the reaction is completed, the slurry is discharged from the bottom of the reactor and enters a 10m... 3 The intermediate liquid tank is pumped to a box filter press for solid-liquid separation; the resulting filter residue is crude selenium residue, which is sent to the crude selenium refining workshop for processing; part of the resulting filtrate is returned to the batching section, and part is used as qualified leachate to enter the reduction section.

[0032] (2) Leachate reduction of crude selenium: The qualified leachate obtained in step (1) is pumped to a 3m... 3 In the reduction reactor, sodium sulfite crystals were slowly added at 40°C with stirring to reduce H2SeO3 in the solution; the actual amount of sodium sulfite crystals added was 1.15 times the amount theoretically required for the complete reduction of H2SeO3 to elemental Se; excess SO2 gas overflowed from the liquid surface and entered the alkaline absorption tower for treatment with acid mist; after 1.0 h of reaction, the slurry was pumped into a box filter press for filtration; the obtained filter residue was mainly a mixture of elemental Se and iron salts, which was sent to the crude selenium refining workshop for treatment together with the crude selenium residue obtained in step (1); the obtained filtrate was the deselenized liquid.

[0033] (3) Neutralization and mercury precipitation: The deselenized liquid is discharged into the reactor, and NaOH tablets are slowly added to adjust the pH of the solution to 6.5. After the neutralization residue is generated, it is centrifuged and filtered, and the neutralization residue is returned to the batching section; Hg in the hydrazine hydrate liquid reduction solution is slowly added dropwise to the neutralized liquid. 2+ This produces water-insoluble mercurous chloride (Hg₂Cl₂); the amount of hydrazine hydrate added is strictly controlled to reduce Hg₂Cl₂ content to a minimum. 2+ The amount of mercury-free liquid is reduced to 1.0 times the theoretically calculated amount of Hg2Cl2; after centrifugation and filtration, the resulting mercury-free liquid is recycled back to the batching section.

[0034] (4) Alkaline leaching of crude selenium slag: The crude selenium slag collected in steps (1) and (2) is added to the leaching stirring tank in the crude selenium refining workshop. The deselenized liquid obtained by centrifugation in the selenium precipitation section is used as the mother liquor. NaOH is added to adjust the mass concentration of NaOH in the final solution to 10%. Hydrazine hydrate is slowly added dropwise at room temperature and pressure. The amount of hydrazine hydrate added is to reduce elemental selenium to Se. 2- The amount was 1.05 times the theoretically required amount; after stirring and reacting for 1.5 hours, pressure filtration was performed. The resulting filter residue contained lead, and the resulting filtrate contained Se. 2- The alkaline leachate.

[0035] (5) Oxidation precipitation of selenium: The selenium-containing product obtained in step (4) is precipitated by oxidation. 2- The leachate was pumped to 4.5m. 3 In a selenium precipitation tank, hydrogen peroxide (H2O2) with a mass fraction of 27.5% was slowly added at room temperature for selenium precipitation treatment; the amount of hydrogen peroxide added was such that the amount of Se... 2- The amount of elemental Se is oxidized to 1.0 times the amount required by theoretical calculation; after reacting for 1.5 hours, the slurry is released and separated by centrifugation and filtration. The resulting solid is the powdered selenium product, and part of the filtrate is returned to step (4) for mother liquor recycling. Example 2

[0036] This embodiment provides a method for the reduction and separation of selenium in the wet leaching solution of acid mud. The raw material, acid mud, is the same as in Example 1, and includes the following steps: (1) Chlorination leaching of acid mud: The raw acid mud is loaded into the leaching reactor in the chlorination workshop, and the circulating bottom liquid containing hydrochloric acid is pumped in at a liquid-to-solid ratio of 3.2:1; stirring is started at 50°C, and sodium chlorate (NaClO3) solid is added slowly in batches for leaching. The amount of sodium chlorate added is 1.10 times the amount required by theoretical calculation, and the leaching reaction time is controlled at 2.0h; after the reaction is completed, the slurry is released and solid-liquid separation is carried out by a box filter press; the obtained filter residue is crude selenium residue, which is sent to the crude selenium refining workshop for processing; part of the obtained filtrate is recycled, and part of it is used as qualified leaching liquid to enter the reduction section.

[0037] (2) Leachate reduction of crude selenium: The qualified leachate obtained in step (1) is pumped to a 3m... 3 In the reduction reactor, sodium sulfite crystals were slowly added at 50°C with stirring. The actual amount of sodium sulfite added was 1.20 times the amount theoretically required for the complete reduction of H2SeO3 to elemental Se. Excess SO2 gas overflowed from the liquid surface and was absorbed by the alkali solution. After reacting for 1.5 hours, the mixture was filtered. The resulting filter residue and the crude selenium residue obtained in step (1) were sent to the crude selenium refining workshop for processing. The resulting filtrate was the deselenized liquid.

[0038] (3) Neutralization and mercury precipitation: After the selenium-depleted liquid is discharged into the reactor, NaOH tablets are slowly added to adjust the pH of the solution to 7.0. After the neutralization residue is generated, it is centrifuged and filtered, and the neutralization residue is returned to the batching section; Hg in the hydrazine hydrate liquid reduction solution is slowly added dropwise to the neutralized liquid. 2+ Mercurous chloride (Hg₂Cl₂) is generated; the amount of hydrazine hydrate added is strictly controlled to reduce the amount of Hg. 2+ The amount of mercury removed is reduced to 1.05 times the theoretically calculated amount of Hg2Cl2; after centrifugation and filtration, the mercury-free liquid is recycled back to the batching section.

[0039] (4) Alkaline leaching of crude selenium slag: The crude selenium slag collected in steps (1) and (2) is added to the leaching stirring tank in the crude selenium refining workshop. The deselenized liquid is used as the mother liquor. NaOH is added to adjust the mass concentration of NaOH in the final solution to 15%. Hydrazine hydrate is slowly added dropwise at room temperature and pressure. The amount of hydrazine hydrate added is to reduce elemental selenium to Se. 2- The amount required was 1.12 times the theoretically calculated amount; after stirring and reacting for 2.0 h, pressure filtration was performed. The resulting filter residue contained lead, and the resulting filtrate contained Se. 2- The alkaline leachate.

[0040] (5) Oxidation precipitation of selenium: The leachate obtained in step (4) is pumped into a selenium precipitation stirring tank, and hydrogen peroxide (H2O2) is slowly added dropwise at room temperature for precipitation treatment; the amount of hydrogen peroxide added is to increase the amount of Se. 2- The amount of elemental Se is oxidized to 1.05 times the amount required by theoretical calculation; after 2.0 h of reaction, it is centrifuged and filtered, and the resulting solid is the powdered selenium product. The filtrate is returned to step (4) for recycling. Example 3

[0041] This embodiment provides a method for the reduction and separation of selenium in the wet leaching solution of acid mud. The raw material, acid mud, is the same as in Example 1, and includes the following steps: (1) Chlorination leaching of acid mud: The raw acid mud is loaded into the leaching reactor in the chlorination workshop, and the circulating bottom liquid containing hydrochloric acid is pumped in at a liquid-to-solid ratio of 4.0:1; stirring is started at 80°C, and sodium chlorate (NaClO3) solid is added slowly in batches for leaching. The amount of sodium chlorate added is 1.15 times the amount required by theoretical calculation, and the leaching reaction time is controlled at 3.0h; after the reaction is completed, solid and liquid are separated; the obtained filter residue is crude selenium residue, which is sent to the refining workshop for processing; the obtained filtrate is used as qualified leaching liquid and enters the reduction section.

[0042] (2) Leachate reduction of crude selenium: The qualified leachate is pumped into the reduction reactor, and sodium sulfite crystals are slowly added at 65°C with stirring. The actual amount of sodium sulfite added is 1.25 times the amount required for complete reduction of H2SeO3 to elemental Se according to theoretical calculation. Excess SO2 gas overflows from the liquid surface and is absorbed by alkaline solution. After reacting for 2.0 h, the mixture is filtered. The resulting filter residue is combined with the crude selenium residue for treatment. The filtrate is the deselenized liquid.

[0043] (3) Neutralization and mercury precipitation: After deselectrophoresis, NaOH tablets are slowly added to the solution to adjust the pH to 8.0. After neutralization residue is generated, it is centrifuged and filtered, and the neutralization residue is returned to the batching section. Hydrazine hydrate liquid is slowly added dropwise to the neutralized solution to generate mercurous chloride (Hg2Cl2). The amount of hydrazine hydrate added is strictly controlled to reduce the amount of Hg2Cl2. 2+ The amount of mercury removed is reduced to 1.10 times the theoretically calculated amount of Hg2Cl2; after centrifugation and filtration, the mercury-free liquid is recycled back to the batching section.

[0044] (4) Alkaline leaching of crude selenium slag: The crude selenium slag is added to the leaching stirring tank in the refining workshop, and mixed with mother liquor. NaOH is added to adjust the mass concentration of NaOH in the final solution to 20%. Hydrazine hydrate is slowly added dropwise at room temperature and pressure. The amount of hydrazine hydrate added is to reduce elemental selenium to Se. 2- The amount was 1.20 times the theoretically required quantity; after stirring and reacting for 2.5 hours, the mixture was separated by pressure filtration. The filter residue contained lead, and the filtrate contained Se. 2- The alkaline leachate.

[0045] (5) Selenium oxidation precipitation: Selenium-containing... 2- The leachate was pumped into a selenium precipitation tank, and hydrogen peroxide (H2O2) was slowly added dropwise at room temperature for selenium precipitation treatment; the amount of hydrogen peroxide added was such that the amount of Se... 2- The amount of elemental Se is oxidized to 1.15 times the amount required by theoretical calculation; after 2.5 h of reaction, it is centrifuged and filtered, and the resulting solid is the powdered selenium product. The filtrate is returned to step (4) for recycling.

[0046] Comparative Examples 1-5: Comparative Example 1: Compared with Example 2, the difference is that in the step (2) leaching solution reduction of crude selenium section, sodium sulfite crystals are not used, but an equivalent amount of conventional iron powder is used as a reducing agent to reduce H2SeO3 in the solution. The rest are the same.

[0047] Comparative Example 2: Compared with Example 2, the difference is that in step (4) of the alkaline leaching section of the crude selenium slag, the alkaline leaching system of hydrazine hydrate and NaOH is not used. Instead, conventional sodium sulfite (Na2SO3) aqueous solution is used to reversibly leach the crude selenium slag at high temperature. All other aspects are the same.

[0048] Comparative Example 3: Compared with Example 2, the difference is that in step (5) of the selenium oxidation precipitation process, hydrogen peroxide (H2O2) is not used as an oxidant for selenium precipitation. Instead, air is continuously blown into the leachate for natural oxidation precipitation. All other aspects are the same.

[0049] Comparative Example 4: Compared with Example 2, the difference lies in the fact that in step (4) of the alkaline leaching section of the crude selenium slag, the amount of hydrazine hydrate liquid added is insufficient, and its addition amount is set to reduce elemental selenium to Se. 2- The amount is 0.8 times the theoretically required amount, and all other quantities are the same.

[0050] Comparative Example 5: Compared with Example 2, the difference is that in step (4) of the alkaline leaching section of the crude selenium slag, NaOH is not added to adjust the alkalinity of the solution (i.e., no strongly alkaline environment is provided), and only hydrazine hydrate is directly added to the mother liquor for leaching. The rest are the same.

[0051] Test Examples 1-5: Test Example 1: This test case involves the detection and quantitative analysis of the main elements in the intermediate solid products and final products produced at various process nodes in Examples 1 to 3.

[0052] During the process of Examples 1 to 3, crude selenium slag samples were collected in the filtration separation stage of the chlorination workshop, lead slag samples were collected in the filtration separation stage of the crude selenium refining workshop, and the final powdered selenium product after centrifugation and washing was collected.

[0053] The collected solid samples were placed in a vacuum drying oven and dried at 60°C until constant weight. The dried samples were then ground in an agate mortar and pestle until they all passed through a 200-mesh standard sieve.

[0054] Accurately weigh 0.2000g of each of the above-treated samples and place them separately in a polytetrafluoroethylene digestion vessel. Add a mixed acid system consisting of nitric acid, hydrochloric acid, and hydrofluoric acid for microwave-assisted closed digestion. After digestion and acid removal, bring the volume to 100mL with deionized water. For high-purity final powdered selenium products, the content of the main component is simultaneously determined by sodium thiosulfate volumetric titration.

[0055] The concentrations of Se, Hg, and Pb in each digestion solution were determined using inductively coupled plasma atomic emission spectrometry (ICP-AES). The mass fraction of the corresponding element in the solid product at each node was calculated based on the fixed volume and sample mass. Each group of samples was independently measured in triplicate, and the average value was recorded.

[0056] Table 1. Test results of the mass fraction of core elements in the solid products of key nodes in Examples 1-3 Example number Hg mass fraction (%) in crude selenium slag Se mass fraction (%) in crude selenium slag Pb mass fraction (%) in lead slag Se mass fraction (%) in lead slag Se mass fraction (%) in powdered selenium products Example 1 0.083 9.14 65.41 0.17 99.12 Example 2 0.047 9.42 67.83 0.09 99.56 Example 3 0.031 9.67 68.21 0.05 99.81 Based on the data in Table 1 and the corresponding... Figure 1 Examples 1 to 3 all successfully separated associated elements from acid sludge within the set parameter range. Figure 1 (a) (a gray-scale bar graph comparing the trace mercury impurity content in crude selenium slag and the trace selenium residue content in lead slag under different process parameters) shows that the mass fraction of Hg in the crude selenium slag produced in the chlorination leaching section is only 0.083%, which is much lower than the initial content in the original acid sludge. This indicates that in the oxidation system composed of hydrochloric acid and sodium chlorate, the mercury-like solid components in the acid sludge are fully oxidized into soluble ions and enter the liquid phase, achieving initial solid-liquid separation of mercury from the crude selenium skeleton.

[0057] from Figure 1 (b) (which is a gray-scale bar graph comparing the enrichment content of selenium in crude selenium slag and lead in lead slag under different process parameters) can be further seen that after alkaline leaching treatment in the crude selenium refining workshop, the Pb mass fraction in the solid residue lead slag reaches 65.41% to 68.21%, indicating that lead compounds did not undergo a dissolution reaction in the acidic chlorination system at the front end and the strongly alkaline hydrazine hydrate reduction system at the back end, and were retained as a stable framework in the final waste residue.

[0058] In addition, such as Figure 1 (c) (a line graph showing the mass fraction of the final precipitated selenium powder under different process parameters) shows that the mass fraction of Se in the final precipitated selenium powder is above 99.12%, confirming that hydrazine hydrate has a highly selective reducing and dissolving effect on elemental selenium in the alkaline environment provided by sodium hydroxide. The solid phase selenium is converted into easily soluble selenium anions, which are separated from insoluble impurities such as lead slag. The subsequent hydrogen peroxide precipitation process causes selenium ions to re-aggregate into elemental powder, verifying the effectiveness of staged redox potential control for the separation of complex components.

[0059] Test Example 2: This test case quantitatively measures the liquefaction reduction conversion efficiency, crystallization efficiency of the precipitation section, and direct material recovery rate of the crude selenium refining system in Examples 1 to 3.

[0060] The mercurous chloride precipitate obtained by centrifugation in the mercury precipitation stage of Examples 1 to 3, and the powdered selenium precipitate obtained by centrifugation in the selenium precipitation stage were collected. The collected solid materials were placed in an explosion-proof vacuum drying oven and dried at 50°C until constant weight. The total mass of each group of solids after drying was weighed separately as the basis for calculating the product recovery rate.

[0061] Weigh 0.5000 g of the dried mercurous chloride sample and place it in a beaker. Add a mixture of nitric acid and sulfuric acid and heat to digest. After cooling, oxidize the low-valence substances in the liquid phase with potassium permanganate solution. Excess potassium permanganate is destroyed by sodium nitrite, and urea is added to decompose excess sodium nitrite. Then, titrate with ferrous ammonium sulfate standard solution to calculate the true mass fraction of mercurous chloride in the solid product.

[0062] 100 mL each of the filtration liquid from the alkaline leaching stage of the crude selenium slag and the centrifuged mother liquor from the selenium oxidation precipitation stage were collected. The liquid samples were serially diluted to a final volume, and the total selenium ion concentration in the liquid system at different stages was determined using inductively coupled plasma atomic emission spectrometry (ICP-AES).

[0063] The total selenium recovery rate for the entire process is calculated by combining the total mass of the final solid selenium powder with the initial total mass of selenium in the original batch of acid sludge. The alkaline leaching rate of crude selenium is calculated using the ratio of the total concentration of selenium ions in the liquid phase to the total selenium content in the crude selenium slag. The single-step crystallization rate of selenium precipitation is calculated by back-calculating the residual selenium concentration in the centrifuged mother liquor.

[0064] Table 2. Key product recovery and conversion indicators and by-product purity test results in Examples 1-3 Example number Crude selenium alkaline leaching rate (%) Full-process selenium direct recovery rate (%) Purity (%) of mercurous chloride byproduct Selenium precipitation single-step crystallization rate (%) Example 1 94.26 91.15 96.33 98.74 Example 2 97.58 94.32 98.12 99.41 Example 3 96.81 93.67 97.45 99.15 Based on the data in Table 2 and the corresponding... Figure 2 , combined Figure 2(a) (a gray-scale bar chart comparing the alkaline leaching rate of crude selenium and the single-step crystallization rate of selenium precipitation under different process parameters) shows that the alkaline leaching rate of crude selenium in Examples 1 to 3 ranges from 94.26% to 97.58%, and the single-step crystallization rate of selenium precipitation ranges from 98.74% to 99.41%. Adding hydrazine hydrate to the alkaline system constructed with sodium hydroxide causes electron transfer in the zero-valent elemental selenium in the crude selenium slag, generating soluble selenium anions that enter the liquid phase. Test data shows that the leaching rate and direct recovery rate are highest under the parameters of Example 2. In Example 3, when the reagent ratio reaches its upper limit, various conversion indicators show a slight decrease. The reason for this reaction is that excess hydrazine hydrate decomposes in the strongly alkaline solution to produce nitrogen gas. The bubbles adhere to the surface of the solid-phase crude selenium particles, forming a gas-solid isolation layer that hinders the solid-liquid interface mass transfer process. Furthermore, the excess reducing agent triggers local reverse side reactions that consume the effective substances.

[0065] In the by-product recycling process, such as Figure 2 (b) (a grayscale line graph showing the direct selenium recovery rate and mercurous chloride by-product purity under different process parameters) shows that the mercurous chloride purity obtained in the mercury precipitation stage ranges from 96.33% to 98.12%. After selenization, hydrazine hydrate in the liquid directionally reduces divalent mercury ions to a monovalent state, forming an insoluble mercurous chloride precipitate. In Example 3, the mercurous chloride purity decreased compared to Example 2 because the reduction potential of hydrazine hydrate at the upper limit of the parameters was too high, causing some divalent mercury ions to bypass the monovalent stage and be directly and deeply reduced to zero-valent metallic mercury. This elemental mercury doping in the precipitate reduced the proportion of single-phase mercurous chloride. In subsequent stages, hydrogen peroxide at room temperature and pressure reverse-oxidizes the selenium anions in the alkaline mother liquor, removing electrons and recrystallizing into elemental selenium powder. Throughout the entire process test, the direct selenium recovery rate remained between 91.15% and 94.32%, verifying the effectiveness of the chemical reaction pathway of alkaline liquefaction of hydrazine hydrate combined with hydrogen peroxide oxidation precipitation.

[0066] Test Example 3: This test example compares and determines the purity and content of key impurity elements in the final powdered selenium products of Example 2, Comparative Example 1, and Comparative Example 2.

[0067] At the end of the process flow, the final powdered selenium products after centrifugation and washing in Example 2, Comparative Example 1, and Comparative Example 2 were collected respectively. The three groups of solid materials were spread evenly in petri dishes, placed in a vacuum drying oven, and dried at 60°C until the difference between two consecutive weighings was less than 0.002g, thus obtaining constant weight samples.

[0068] Accurately weigh 0.1500 g of each of the three dried samples and transfer them to a polytetrafluoroethylene digestion vessel. Add a mixture of concentrated nitric acid and perchloric acid to the digestion vessel and place it in a microwave digester to perform programmed temperature digestion. After the digestion solution becomes clear and transparent, start the venting and acid removal program. After cooling, transfer the solution to a 50 mL volumetric flask and dilute to volume with deionized water.

[0069] Inductively coupled plasma atomic emission spectrometry (ICP-AES) was used to select analytical spectral lines and determine the detection concentrations of iron and lead in each group of diluted digestion solutions. The mass fractions of iron and lead impurities in the product were calculated based on the diluted volume, dilution factor, and sample mass.

[0070] Three additional 0.2000g samples were dried and dissolved in nitric acid, with urea used to eliminate interference from nitrogen oxides. Potassium iodide was then added to reduce excess oxygen, followed by redox titration using sodium thiosulfate standard titration solution. The volume of standard solution consumed was recorded, and the total mass fraction of the powdered selenium product was calculated. All tests were performed in triplicate.

[0071] Table 3. Test results of purity and impurity content of the final products of Example 2, Comparative Example 1, and Comparative Example 2 Group numbering Total selenium mass fraction (%) in powdered selenium products Iron (Fe) mass fraction (%) in the product Lead (Pb) mass fraction (%) in the product Example 2 99.56 0.004 0.007 Comparative Example 1 83.29 12.65 0.014 Comparative Example 2 91.18 0.006 4.38 Based on the data in Table 3 and the corresponding... Figure 3 , combined Figure 3 (a) (which is a grayscale bar chart comparing the total selenium mass fraction in the final powder selenium product under different reagents and process systems) shows that the final product of Example 2 has a total selenium mass fraction of 99.56%, and as Figure 3 (b) (which is a gray-scale clustered columnar comparison of the mass fraction of iron and lead impurities in the final powder selenium product under different reagents and process systems) shows that the retention rates of iron and lead impurities are both less than 0.01%.

[0072] In Comparative Example 1, iron powder was used instead of sodium sulfite as the first reducing agent. The total selenium mass fraction in the final product decreased to 83.29%, while the iron impurity mass fraction increased to 12.65%. This is because the iron powder, after participating in the reduction reaction, generates a large amount of soluble iron ions. These iron ions, along with the crude selenium slag, enter the strongly alkaline environment of the subsequent crude selenium refining workshop, reacting with sodium hydroxide to form ferric hydroxide colloidal precipitate with flocculation properties. This ferric hydroxide colloid not only encapsulates the solid-phase crude selenium particles, hindering the mass transfer of hydrazine hydrate to the solid-liquid interface, but it is also difficult to be effectively intercepted by the filter cloth during the pressure filtration separation stage. It then enters the final oxidation precipitation stage with the deselection mother liquor, resulting in a large proportion of iron impurities being mixed into the final product.

[0073] In Comparative Example 2, conventional sodium sulfite reversible leaching was used to replace the alkaline leaching system of hydrazine hydrate. The product contained 91.18% total selenium and 4.38% lead impurities. This is because the dissolution of elemental selenium by sodium sulfite is an endothermic and reversible reaction, requiring a high-temperature boiling environment to maintain the forward reaction. Under the continuous high temperature and high-concentration salt system, the stable lead compounds in the original crude selenium slag partially dissociated, forming lead complexes that entered the leachate. In subsequent cooling or precipitation stages, the dissolved lead phase recrystallized and precipitated, mixing with the powdered selenium and causing lead contamination.

[0074] This scheme cuts off the introduction pathway of exogenous metal cations by using sulfite, which either overflows in gaseous form or generates soluble anions, as a reagent in the reduction section; and by utilizing the strong reducing properties of hydrazine hydrate in the refining section to complete the unidirectional irreversible conversion of solid-phase selenium to liquid-phase selenium anions at room temperature and pressure, thus avoiding the dissolution of lead phase side reactions caused by high-temperature systems and ensuring the physical separation of multiple impurities from the reaction mechanism perspective.

[0075] Test Example 4: This test case quantitatively analyzes the selenium leaching efficiency and overall direct recovery rate in Example 2, Comparative Example 4, and Comparative Example 5 to verify the necessity of setting the boundary of reducing agent dosage parameters and the strongly alkaline environment.

[0076] During the process operation, crude selenium slag produced in the chlorination leaching section of Example 2, Comparative Example 4 and Comparative Example 5, and lead slag produced after alkaline leaching and pressure filtration in the crude selenium refining workshop were collected, and the final powdered selenium product was collected.

[0077] The collected solid materials from each stage were placed in a forced-air drying oven and dried to constant weight at 60℃. 0.2500g each of the dried crude selenium slag and lead slag samples were weighed and placed separately in polytetrafluoroethylene digestion vessels. Nitric acid, hydrochloric acid, and hydrofluoric acid were added for microwave-assisted closed digestion. After removing the acid, the volume was adjusted to 100mL with deionized water.

[0078] The total selenium mass concentration in each digestion solution was detected by inductively coupled plasma atomic emission spectrometry, and the initial and residual selenium mass fractions in the corresponding solid materials were calculated.

[0079] Record the total mass of the crude selenium slag input and the total mass of the lead slag output for each batch. Combine this with the mass fractions detected above to calculate the crude selenium alkaline leaching rate. Simultaneously, weigh the total mass of the final dried selenium powder product. Combine this with the initial selenium content in the original batch of acid sludge to calculate the overall selenium direct recovery rate. Each experiment and test was performed in triplicate, and the arithmetic mean was taken.

[0080] Table 4. Test results of selenium leaching and recovery indicators in Example 2, Comparative Examples 4 and 5 Group numbering Total selenium mass fraction (%) in crude selenium slag Residual selenium mass fraction (%) in lead slag Crude selenium alkaline leaching rate (%) Full-process selenium direct recovery rate (%) Example 2 9.42 0.09 97.58 94.32 Comparative Example 4 9.38 3.14 68.35 62.14 Comparative Example 5 9.45 8.27 14.27 11.53 Based on the data in Table 4 and the corresponding... Figure 4 , combined Figure 4 (b) (a gray-scale clustered columnar graph comparing the crude selenium alkaline leaching rate and the overall selenium direct recovery rate under different reagents and process systems) shows that the crude selenium alkaline leaching rate in Example 2 reached 97.58%, and the overall selenium direct recovery rate was 94.32%. Meanwhile, from... Figure 4 (a) (which is a gray-scale clustered columnar comparison of the mass fraction of total selenium in crude selenium slag and residual selenium in lead slag under different reagents and process systems) It can be seen that the mass fraction of residual selenium in lead slag is only 0.09%.

[0081] In Comparative Example 4, when the amount of hydrazine hydrate added was reduced to 0.8 times the theoretically calculated amount, the alkaline leaching rate of crude selenium decreased significantly to 68.35%, and the residual selenium mass fraction in the lead slag increased to 3.14%. This is because the total amount of electron donors provided in the system was insufficient to cover the reduction requirements of all elemental selenium in the crude selenium slag. Consequently, some solid-phase zero-valent selenium could not obtain enough electrons to convert into selenium anions. Elemental selenium that did not undergo phase transfer was retained in the lead slag along with insoluble impurities, resulting in a decrease in both the front-end leaching and back-end direct recovery indicators.

[0082] In Comparative Example 5, without adding sodium hydroxide to adjust the solution alkalinity, only hydrazine hydrate was added dropwise to the mother liquor. This resulted in a significant drop in the alkaline leaching rate of crude selenium, to only 14.27%, and a total direct selenium recovery rate as low as 11.53%, with a residual selenium mass fraction in the lead slag reaching as high as 8.27%. This is because the redox potential of hydrazine hydrate is dependent on the pH of the system. In the absence of a strongly alkaline environment, the reducing power of hydrazine hydrate is greatly weakened, failing to overcome the thermodynamic energy barrier of elemental selenium's conversion to divalent selenium ions. Furthermore, the electron transfer process at the solid-liquid interface is slow, and elemental selenium does not meet the dissolution conditions at room temperature.

[0083] This scheme improves the reducing power of hydrazine hydrate by limiting the excess ratio boundary of hydrazine hydrate and adding sodium hydroxide to construct an alkaline system, thus ensuring the liquefaction and separation of zero-valent selenium.

[0084] Test Example 5: This test case measures the phase transformation index and liquid phase residue of Examples 2, 3 and Comparative Example 3 in the oxidation precipitation section.

[0085] The solid-liquid mixture slurry from the selenium-depleted liquids of Examples 2, 3, and Comparative Example 3 after oxidation precipitation reaction was collected. Solid-liquid separation was performed using a centrifuge at 3000 r / min, and the supernatant mother liquor and the bottom solid precipitate were collected separately.

[0086] The solid precipitate was washed three times alternately with deionized water and anhydrous ethanol, and then dried in a vacuum drying oven at 55°C until constant weight. The total mass of the dried powder was weighed, and the single-step crystallization rate of selenium precipitation was calculated by combining it with the initial total selenium mass in the corresponding batch of alkaline leaching mother liquor.

[0087] Weigh 0.2000 g of the dried powder sample, add nitric acid and hydrofluoric acid, and perform closed digestion in a microwave digester. After adjusting the volume, the concentration of total selenium, the main component, in the digestion solution is determined by inductively coupled plasma atomic emission spectrometry (ICP-AES), and the purity of the powder product is calculated.

[0088] Take 50 mL of the supernatant mother liquor obtained from centrifugation of each group, and use a potentiometric titrator equipped with a platinum ring indicator electrode and a silver-silver chloride reference electrode to determine the endpoint potential of the oxidation precipitation at room temperature.

[0089] Take 10 mL of the supernatant mother liquor sample and dilute it stepwise with 2% nitric acid solution. Detect the residual total selenium concentration in the liquid phase system using an atomic fluorescence spectrophotometer. Each of the above tests was performed in triplicate, and the arithmetic mean was calculated.

[0090] Table 5. Oxidation precipitation index and liquid phase residue test results of Examples 2, 3 and Comparative Example 3 Group numbering Oxidation precipitation endpoint potential (mV) Selenium precipitation single-step crystallization rate (%) Residual selenium concentration in the mother liquor (mg / L) Powder product purity (%) Example 2 -84.5 99.41 18.63 99.56 Example 3 -21.3 99.15 145.27 99.81 Comparative Example 3 -328.6 41.15 7851.34 95.22 Based on the data in Table 5 and the corresponding... Figure 5 , combined Figure 5 (a) (a gray-scale clustered columnar graph comparing the single-step crystallization rate of selenium precipitation and the purity of powder products under different potential control systems) shows that in Example 2, under the controlled hydrogen peroxide oxidation system, the precipitation endpoint potential was maintained at -84.5mV, and the single-step crystallization rate of selenium precipitation reached 99.41%, and as... Figure 5 (b) (which is a logarithmic grayscale line graph of the residual selenium concentration in the mother liquor under different potential control systems) shows that the residual selenium concentration in the mother liquor decreased to 18.63 mg / L, and the purity of the powder product was 99.56%. In Example 3, the hydrogen peroxide ratio reached the upper limit of the parameter, the endpoint potential shifted to -21.3 mV, the crystallization rate decreased slightly to 99.15%, and the residual selenium concentration in the mother liquor increased to 145.27 mg / L.

[0091] The reason is that when the redox potential of the system approaches the zero potential limit, a local peroxidation reaction occurs, and a small amount of zero-valent elemental selenium that has already crystallized loses electrons and transforms into high-valent oxyanion ions such as selenite. These high-valent ions have high solubility in alkaline solutions, and their re-entry into the liquid phase causes a trace amount of selenium to dissolve back into the liquid. In Comparative Example 3, room temperature air aeration was used instead of hydrogen peroxide as the oxidant, and the endpoint potential remained at -328.6 mV, with a single-step crystallization rate of only 41.15%, and the residual selenium concentration in the mother liquor reached 7851.34 mg / L. This is because oxygen has low solubility in strongly alkaline aqueous solutions at normal pressure, and the gas-liquid mass transfer resistance hinders the electron exchange rate. The oxidation potential provided by the liquid phase cannot effectively overcome the kinetic energy barrier of the transformation of selenium anions into zero-valent elemental selenium. The divalent selenium ions do not undergo sufficient electron removal reaction and continue to exist in the mother liquor in a dissolved state, resulting in a decrease in the proportion of solid phase precipitation.

[0092] This scheme utilizes the homogeneous dispersion characteristics of hydrogen peroxide in the liquid phase. By controlling the amount of hydrogen peroxide added, the endpoint potential of oxidation precipitation is limited to the micro-reduction range, which blocks the thermodynamic side reaction pathway of the evolution of selenium anions into high-valence selenate ions, thus ensuring the crystallization and precipitation of the target product.

Claims

1. A method for the reduction and separation of selenium in wet leaching solution of acid mud, characterized in that, Includes the following steps: Chlorination leaching steps for acid mud: Mix raw acid mud with hydrochloric acid-containing bottom liquid, add sodium chlorate solid to react, and separate the solid and liquid to obtain crude selenium slag and qualified leachate; Leachate reduction of crude selenium step: Sodium sulfite crystals are added to the qualified leachate for reaction, and the mixture is filtered to obtain crude selenium residue and deselenized liquid; Neutralization and mercury precipitation steps: NaOH is added to the selenium-depleted liquid to adjust the pH and produce neutralization residue. After filtration, hydrazine hydrate liquid is added dropwise to the neutralized liquid to react. After filtration, mercury precipitation residue and mercury-depleted liquid are obtained. Alkaline leaching step of crude selenium slag: Combine the crude selenium slags obtained from the previous two steps and mix them with the bottom solution. Add NaOH to adjust the mass concentration of NaOH in the final solution to 10% to 20%. Add hydrazine hydrate solution dropwise at room temperature and pressure. The amount of hydrazine hydrate solution added is controlled to reduce the elemental selenium in the crude selenium slag to Se. 2- The amount of Se was increased to 1.05 to 1.20 times the theoretical required amount, and the reaction was stirred for 1.5 to 2.5 hours, followed by pressure filtration to obtain Se-containing products. 2- The alkaline leachate; Oxidation precipitation of selenium: [The process involves adding selenium to the product containing Se...] 2- Hydrogen peroxide was slowly added dropwise to the alkaline leachate for precipitation treatment. The amount of hydrogen peroxide added was controlled to reduce the amount of Se... 2- The amount of elemental selenium is oxidized to 1.0 to 1.15 times the theoretical amount required for elemental selenium, and then centrifuged to obtain powdered selenium product and filtrate.

2. The method for reduction and separation of selenium in wet leaching solution of acid mud according to claim 1, characterized in that, In the alkaline leaching step of the crude selenium slag, NaOH is added to adjust the mass concentration of NaOH in the final solution to 15%, and the amount of hydrazine hydrate added is controlled to reduce the elemental selenium in the crude selenium slag to Se. 2- 1.12 times the theoretically required amount.

3. The method for reduction and separation of selenium in wet leaching solution of acid mud according to claim 1, characterized in that, The acid sludge chlorination leaching step includes loading the raw acid sludge into the leaching reactor, pumping the hydrochloric acid-containing bottom liquid as the circulating bottom liquid at a liquid-to-solid ratio of 2.5:1 to 4.0:1, and starting stirring at room temperature to 80°C.

4. The method for reduction and separation of selenium in wet leaching solution of acid mud according to claim 3, characterized in that, In the acid mud chlorination leaching step, the sodium chlorate solid is added slowly in batches for leaching. The amount of sodium chlorate solid added is controlled to be 1.05 to 1.15 times the theoretically required amount, and the leaching reaction time is controlled to be 1.5 h to 3.0 h.

5. The method for reduction and separation of selenium in wet leaching solution of acid mud according to claim 1, characterized in that, The step of reducing crude selenium with leachate includes pumping the qualified leachate into a reduction reactor and slowly adding the sodium sulfite crystals at 40°C to 65°C with stirring to carry out the reduction reaction.

6. The method for reducing and separating selenium in wet leaching solution of acid mud according to claim 5, characterized in that, In the step of reducing crude selenium with leachate, the amount of sodium sulfite crystals added is controlled to be 1.15 to 1.25 times the theoretical amount required to completely reduce H2SeO3 in the solution to elemental Se. After reacting for 1.0 to 2.0 hours, solid-liquid separation is performed.

7. The method for reduction and separation of selenium in wet leaching solution of acid mud according to claim 1, characterized in that, The neutralization and mercury precipitation steps include draining the selenium-depleted liquid into a reaction vessel, slowly adding NaOH tablets to adjust the pH of the solution to 6.5 to 8.0, and centrifuging and filtering the resulting neutralized residue to obtain the neutralized liquid.

8. The method for reduction and separation of selenium in wet leaching solution of acid mud according to claim 7, characterized in that, Slowly add Hg from the hydrazine hydrate liquid reducing solution to the neutralized solution. 2+ Hg₂Cl₂ precipitate is generated, and the amount of hydrazine hydrate liquid added is controlled to reduce the Hg in the neutralized solution. 2+ Reduce to 1.0 to 1.10 times the theoretically required amount of Hg2Cl2.

9. The method for reducing and separating selenium in wet leaching solution of acid mud according to claim 1, characterized in that, In the selenium oxidation precipitation step, the Se-containing... 2- Selenium precipitation treatment was carried out by adding 27.5% hydrogen peroxide by mass to the alkaline leachate, and the reaction time was controlled to be 1.5h to 2.5h.

10. The method for reduction and separation of selenium in wet leaching solution of acid mud according to claim 1, characterized in that, The filtrate generated during the oxidation and precipitation of selenium is produced by centrifugation. Part or all of the filtrate is recycled back to the alkaline leaching step of the crude selenium residue as a base liquid.

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